"Observing these elusive remnants that may date from the formation of our solar system as they come close to Earth is expanding our understanding of our world and the space it resides in," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate. "Closer study of these objects challenges our capabilities for future exploration and will help us test ways to protect our planet from impact."

NASA scientists have been working on a plan to use robotics to get close enough to study an asteroid some time in the 2020s. By capturing an asteroid and examining its makeup, scientists hope to better prepare to send humans to Mars.

The space agency said its scientists are focusing on two different concepts. One entails capturing a "very small asteroid" in open space, the other envisions collecting a boulder-sized sample off a much larger asteroid.

NASA is expected to decide which course to take sometime this year.

Both concepts call for the robotic spacecraft to redirect an asteroid less than 32 feet in size into the moon's orbit.

Once the asteroid is in a steady orbit, a team of astronauts would blast off in an Orion spacecraft atop a heavy lift rocket. Once in space, the crew would set off on an expected nine-day journey to the asteroid. After docking the spacecraft with the robotic capture vehicle, astronauts would conduct spacewalks to explore the asteroid and take samples.

"With these system concept studies, we are taking the next steps to develop capabilities needed to send humans deeper into space than ever before, and ultimately to Mars, while testing new techniques to protect Earth from asteroids," said William Gerstenmaier, associate administrator for NASA's Human Exploration and Operations Mission Directorate.

NASA has been looking to launch a plan to capture a near-Earth asteroid that could weigh as much as 500 tons. Engineers expect it could happen as early as 2021.

Astronomers used NASA's Spitzer Space Telescope to spot a asteroid, dubbed 2011 MD. So far, it appears to be a good candidate for capture. But the space agency continues to look for others.

So far, nine asteroids have been identified as potential candidates for the mission.

"This mission represents an unprecedented technological feat that will lead to new scientific discoveries and technological capabilities and help protect our home planet," NASA Administrator Charles Bolden had said in an earlier statement. "We will use existing capabilities, such as the Orion crew capsule and Space Launch System rocket, and develop new technologies like solar electric propulsion and laser communications -- all critical components of deep space exploration."

The $800 million mission, which will call on a robot to collect pieces of an asteroid -- named 1999 RQ36 -- will be the first U.S. mission to carry asteroid samples back to Earth. The spacecraft is scheduled to reach the asteroid, which is about the size of five football fields, by 2020 and then return to Earth with samples in 2023.

Asteroids are leftovers formed from the cloud of gas and dust that collapsed to form our sun and the planets about 4.5 billion years ago. Scientists calculate that they contain original planet- and star-forming material, which they hope can tell us about the conditions of our solar system's birth.

Since asteroids are thought to have changed little over time, they likely represent a snapshot of our solar system's infancy.